If you need a tree in 10, maybe don't plant one that takes 20 years to grow.

If nuclear were the only option, it would be different. But the only pro-nuclear policies that make sense for achieving 1.5-2 degrees are keeping existing plants online and reactivating old ones (also policies with reasonable disagreements, but at least positive regarding climate change afaict). Building new ones simply takes too long to achieve climate goals while wind & PV are reasonably quick to do (if the political will is there).